1. Field of the Invention
The present invention relates to an image reading apparatus and method for correcting a pixel reading position.
2. Description of the Related Art
Conventionally, when a data amount communicated during scanning exceeds a communicable data amount or a personal computer cannot process target data during scanning, scanning temporarily stops until data in a predetermined buffer capacity are processed. This mode will be called start-stop.
When a start-stop occurs, an optical carriage returns by a predetermined distance in a direction (to be referred to as backward) opposite to the scan direction, and then stops. The optical carriage is often moved backward because scanning needs to be done at a predetermined speed and the optical carriage requires an approach distance to reach that speed. Depending on the scan mode, the optical carriage is not moved backward. After a stop, image data communication and data processing progress, and a predetermined image data buffer becomes available. Then, the optical carriage operates in the scan direction (to be referred to as a forward direction), and starts image reading so that a scanned image is connected to one read before the start-stop occurred. That is, reading starts from an image next to the one read when the start-stop occurred.
When the optical carriage stops upon generation of a start-stop, it may slightly overrun an original stop position owing to the backlash of the motor. In this case, if the backward distance equals the forward distance to the reading start position, the optical carriage reads an image from a position advanced by the backlash. The image is reduced by the backlash, generating a streak in the image.
To prevent this, the forward distance to the reading start position is sometimes corrected with respect to the backward distance in consideration of the backlash.
Conventionally, an image reading start position after canceling a start-stop is corrected by the backlash of the motor driving system.
According to Japanese Patent Laid-Open No. 2001-127965, the time difference between the image reading start timing and the exciting timing of a stepping motor for moving the reading position is set equal between an operation before image reading temporarily stops and that when image reading restarts. This arrangement prevents an image streak upon a start-stop.
However, under the influence of a heat source near the optical carriage, optical system parts including a lens and mirror may change during the interval between the timing when a start-stop occurs to stop the optical carriage and that when the start-stop is canceled to start operating the optical carriage. The change may further change the image reading position, reducing or enlarging the image.